1. Field of the Invention
This invention relates to a magnet member for use in a driving device using magnetism such as a motor, or a magnetic device using a magnetic roll, for example, a developing device in an apparatus using a developer to form images, and a method of manufacturing the same, and in particular to a method of magnetic orientation.
2. Related Background Art
Cylindrical resin magnets such as cylindrical isotropic resin magnets and cylindrical radial anisotropically oriented resin magnets for use in stepping motors or the like used in cameras, copying machines and other instruments are known.
In an isotropic resin magnet, magnetic powder in the resin magnet material is mixed in an indefinite direction and the readily magnetizable axes of the particles of the magnetic powder face in an indefinite direction and therefore, the magnet characteristic after magnetization is low.
In contrast, in the radial anisotropically oriented resin magnet as shown in FIG. 1 of the accompanying drawings, the readily magnetizable axes of the particles of the magnetic powder are oriented radially of the cylindrical resin magnet and the magnet characteristic after magnetization is good as compared with that of the isotropic resin magnet.
As a method of manufacturing such a radial anisotropically oriented cylindrical resin magnet, it has most often been the practice to magnetically orient magnetic powder in the radial direction during molding and effect magnetization along the magnetic orientation. This resin magnet magnetically oriented in the radial direction is excellent in industrial productivity and the orientational property thereof can be easily made very high. However, this orientation is in one linear direction and therefore, the orientation does not coincide with the direction of a magnetic flux provided when multipole magnetization is effected on the outer periphery of the magnet. Therefore, the magnetic flux passing through the magnet after magnetization is only in the radial direction and thus, it is magnetically closed in the air outside the magnet wherein the magnetic resistance is great, and the magnet becomes weak.
Further, a polar anisotropically oriented resin magnet exhibiting polar anisotropic orientation relative to the outer peripheral direction of the cylindrical resin magnet as shown in FIG. 2A of the accompanying drawings shown in Japanese Patent Publication No. 5045/1981 in order to improve the magnet characteristic is regarded as good. However, such a polar anisotropically oriented resin magnet according to the prior art has suffered from a disadvantage that the greater the number of poles, the greater the reduction in the degree of orientation of magnetic powder. That is, as the number of poles becomes greater, the main magnetic flux from the magnetic pole for orientation does not pass through the deep part of the molded article but passes through only the surface thereof which is at the shortest distance, and further, the intensity thereof reduces to a fraction of several to several tens as compared with the radial orientation or the axial orientation using opposed magnetic poles. This has led to a defect in principle that the degree of orientation is very much reduced by the combined effect thereof. That is, in an example wherein the number of magnetic poles is eight as shown in FIG. 2B, if magnetic pole members 2 and 4 are disposed at locations situated at poles designated by N and S to provide eight-pole orientation and a magnetic flux is passed from the N pole toward the S pole, the magnetic flux travelling from the N pole member 2 to the S pole member 4 will pass through the magnetic powder particles a, b, c and d in the molding resin material and at the same time, will flow along the outer peripheral surface of the molded article 1 and also pass through a non-magnetic member, not shown, present between the magnetic pole members 2 and 4, thus producing a leakage flux. As a result, the orientation of the magnetic powder near the central portion of the molded article is not so much enhanced in degree of orientation as desired or considered in designing. The degree of orientation in the central portion of the molded article is reduced as the number of magnetic poles is increased from eight to sixteen or thirty-two. Consequently, even if an attempt is made to increase the thickness of the molded article and enhance the magnetic characteristic of the magnet to thereby enhance the working point, the depth of the orientation from the surface layer and the degree of orientation will not change and therefore, in the case of multipole orientation, deep places exist while remaining isotropic and a great improvement is not obtained, and when the magnet is to be used as a high-speed rotating member, only the moment of inertia becomes great with a result that the starting frequency and the maximum response frequency are rather reduced.
Further, when the number of poles is significantly increased to obtain a magnet of high resolving power, there is a disadvantage that making a metal mold becomes very disadvantageous in terms of both space and strength, because the increased number of poles makes the pitch and angle between the magnetic poles smaller. Consequently, when it is desired to make a resin magnet having a number of poles greater than a certain degree, the only available method has been to magnetize a radially oriented article of a low magnetic characteristic and thus a magnet of high torque could not be obtained. As a magnet of high torque, there is one using a sintered radially oriented article of high magnetic characteristic, and such magnet can provide a high resolving power. However it is great in specific gravity as compared with a resin magnet and therefore, a magnet of high-speed rotation and high frequency response cannot be provided.
Also, an isotropic or anisotropic sintered magnet or a resin magnet attached to or forced into a core metal has heretofore been popular as a magnet roller used in a copying machine or other instrument. Also, in recent years, there have been made numerous applications including Japanese Laid-Open Patent Application No. 108207/1981 in which polar anisotropic orientation integral molding is internally effected by the application of Japanese Patent Publication No. 28287/1964 using the injection molding method or the aforementioned Japanese Patent Publication No. 5045/1981.
However, the attachment of the sintered magnets according to the prior art has suffered from such problems as being breakable, requiring a lot of time for adhesion, and high cost. Further, the sintered, integrally molded magnet has also suffered from such problems as low yield and low dimensional accuracy caused by burnout, warping or the like, and has unavoidably suffered from such problems as heavy weight and high cost attributable to secondary working. The attachment of a resin magnet by the use of rubber or plastics has also suffered from such problems as high cost attributable to adhesion, secondary working or the like. Therefore, in recent years, integrally molded articles using resin magnets have also appeared However, in fact, isotropic magnets lack the magnetic force. So, numerous attempts have been made to make magnet rollers anisotropic.